U.S. patent number 3,927,249 [Application Number 05/506,247] was granted by the patent office on 1975-12-16 for explosion proof plastic electrical enclosure.
This patent grant is currently assigned to Appleton Electric Company. Invention is credited to James N. Pearse.
United States Patent |
3,927,249 |
Pearse |
December 16, 1975 |
Explosion proof plastic electrical enclosure
Abstract
An electrical wiring enclosure comprising a box and cover is
formed primarily of plastic. At the juncture of the box and cover
the two have mating metal flanges. The metal flange on the cover is
a unitary part of a metallic member which extends across the inside
of the cover in the area in which switches, etc., are mounted. The
box has threaded metallic inserts to receive electrical conduit.
These are electrically connected to the flange of the box.
Inventors: |
Pearse; James N. (Libertyville,
IL) |
Assignee: |
Appleton Electric Company
(Chicago, IL)
|
Family
ID: |
24013807 |
Appl.
No.: |
05/506,247 |
Filed: |
September 16, 1974 |
Current U.S.
Class: |
174/51; 174/53;
220/88.1 |
Current CPC
Class: |
H02B
1/28 (20130101); H02G 3/088 (20130101); H02B
1/46 (20130101) |
Current International
Class: |
H02B
1/00 (20060101); H02B 1/28 (20060101); H02G
3/08 (20060101); H02B 1/46 (20060101); H02G
003/14 () |
Field of
Search: |
;174/50,51,53
;220/88R,3.2-3.9 ;310/88 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Truhe; J. V.
Assistant Examiner: Tone; David A.
Attorney, Agent or Firm: Darbo, Robertson &
Vandenburgh
Claims
I claim:
1. In an explosion proof electrical enclosure comprising a box and
a cover releasably attached to the box which box and cover have
internal walls defining an internal opening for electrical wiring,
said box and cover have mating faces at the juncture therebetween,
the improvement comprising:
said box and cover being mainly formed of plastic and each
including a metal flange at the respective face and circumscribing
said opening at the face, said flanges being in immediate
juxtaposition when said cover is on said box, said flanges being
formed of an erosion resistant metal.
2. In an explosion proof enclosure as set forth in claim 1, wherein
said faces are at approximately right angles to the inner wall of
the box and have inner and outer parts, said flanges are in
juxtaposition to only the inner part of said faces with the plastic
being exposed at the outer part of said faces, said outer part of
the faces being substantially coplanar with the respective
flanges.
3. In an explosion proof enclosure comprising a box and a cover
releasably attached to the box which box and cover have internal
walls defining an internal opening for electrical wiring, said box
and cover have mating faces at the juncture therebetween, and for
use with grounded metal conduit and an electrical component adapted
to be mounted on the cover and having a metallic mounting member
which is intended to be connected to ground, and having a portion
of the box adapted to receive and engage said conduit and a portion
of the cover adapted to receive and support said component, the
improvement comprising:
said box and cover being mainly formed of plastic and each
including a metal flange at the respective face and circumscribing
said opening at the face, said flanges being in immediate
juxtaposition when said cover is on said box, said flanges being
formed of an erosion resistant metal;
said portion of the box including a first metallic member which is
in electrical contact with said conduit when the conduit is in
engagement with the box;
said portion of the cover including a second metallic member which
is in electrical contact with the mounting member when the
component is mounted on the cover;
first means electrically connecting the first metallic member and
the metal flange of the box, and
second means electrically connecting the second metallic member and
the flange of the cover;
said flanges being in contact with each other thereby forming an
electrical connection therebetween.
4. In an explosion proof enclosure as set forth in claim 3,
wherein
the box flange has an integral portion which is in juxtaposition to
part of the internal wall of the box, said flange portion being
connected to said first metallic member and forming said first
means; and
the cover flange has an integral portion which is in juxtaposition
to part of the internal wall of the cover, said cover flange
portion forming said second metallic member and said second
means.
5. In an explosion proof enclosure as set forth in claim 4, wherein
said first metallic member is annular with internal threads and
ridges on the exterior thereof, said first metallic member being an
insert in said plastic.
6. In an explosion proof enclosure as set forth in claim 3, wherein
said first metallic member is annular with internal threads and
ridges on the exterior thereof, said first metallic member being an
insert in said plastic.
7. In an explosion proof enclosure as set forth in claim 6, wherein
said faces are at approximately right angles to the inner wall of
the box and have inner and outer parts, said flanges are in
juxtaposition to only the inner part of said faces with the plastic
being exposed at the outer part of said faces, said outer part of
the faces being substantially coplanar with the respective
flanges.
8. In an explosion proof enclosure comprising a box and a cover
releasably attached to the box which box and cover have internal
walls defining an internal opening for electrical wiring, said box
and cover have mating faces at the juncture therebetween, said
enclosure being rectangular and having four corners, said cover
being attached to the box by screws and the enclosure being mounted
by means of screws, the improvement comprising:
said box and cover being mainly formed of plastic and each
including a metal flange at the respective face and circumscribing
said opening at the face, said flanges being in immediate
juxtaposition when said cover is on said box, said flanges being
formed of an erosion resistant metal; and
a metallic stud in each of said corners of said box with a top end
in juxtaposition to the box face and an opposite end in
juxtaposition to the opposite side of the box, each of said ends
being threaded to receive said screws.
9. In an explosion proof enclosure as set forth in claim 8, wherein
said faces are at approximately right angles to the inner wall of
the box and have inner and outer parts, said flanges are in
juxtaposition to only the inner part of said faces with the plastic
being exposed at the outer part of said faces, said outer part of
the faces being substantially coplanar with the respective flanges.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
"Explosion proof" electrical boxes or enclosures are intended to
prevent the propagation of any flame that may occur inside the
enclosure from reaching the outside of the enclosure. They are not
hermetically sealed and thus explosive atmospheres which may exist
on the outside of the enclosure can reach the interior of the
enclosure through the comparatively small fluid passageways that
will exist between the box and its cover, etc. These explosive
atmospheres within the enclosure can be ignited, as for example by
the arcing of an electrical switch mounted within the box with the
result that an explosion will occur within the box. The explosive
pressure within the box will be vented out through the same
passageways, but because of the design of the passageways, no flame
reaches the exterior of the box, i.e., the gases passing through
the passageways are cooled sufficiently to stop the passage of
flame. Thus, such boxes can be safely employed in rooms, etc., in
which there are likely to be combustible gases or vapors.
These combustible gases or vapors may also be corrosive to varying
degrees, and the explosion-proof enclosures may thus suffer
deterioration as a result of corrosive attack. Other
non-combustible gases or vapors which are highly corrosive are
frequently present for these applications, which compounds the
problem with respect to corrosive attack of explosion-proof
enclosures. The metals most commonly used as enclosure materials
are cast iron, malleable iron and aluminum, although stainless
steel is occasionally used for very severe corrosive conditions at
a substantial premium in cost.
It would be very desirable to use certain plastics which are
relatively inert and not attackd by the corrosive vapors in some of
the applications described above, particularly certain fiberglass
reinforced polyester resins, but flame joints constructed from
plastic materials suffered deterioration as a result of repeated
explosions within the explosion-proof electrical enclosures.
It must be understood that explosion-proof enclosures must be
designed to withstand repeated explosions without deterioration.
For instance, these enclosures may contain devices with arcing
contacts which may operate several times per minute. Because the
enclosures are not hermetically sealed, explosions will occur as a
result of ignition by the arcing contacts within the box. For a box
constructed with flanged joints, the length of the flame path is
the width of the flanged joint, and the gap of the flame path is
the maximum allowable distance between the mating parts of box and
cover.
If this gap should increase, it would be possible at some flame
path length to gap ratio for flame to propagate to the outside of
the box causing a resultant explosion in the room in which the box
is located which could lead to great damage and possible loss of
life.
Repeated explosions of the type described above have no effect on
the flame path for boxes constructed with metal flanges, whereas
the combination of hot gas at a high velocity through the flame
path of a box constructed with plastic flanged joints will score
the plastic leading to an increase in the effective gap height and
inevitably propagation of flame to the exterior of the box.
Consequently, while it has in the past been possible to construct
an enclosure with the required strength to stand repeated
explosions, and plastic boxes could be constructed to resist the
effects of corrosion in a fashion superior to most metal boxes,
plastic boxes could not safely be used as explosion-proof
enclosures because of the effects of this gap deterioration due to
repeated explosions within the box.
It should also be recognized that electrical enclosures for
application in hazardous locations will be connected to rigid
conduit or other approved wiring means in order that insulated
wires may be used to transfer electrical power to and from the
enclosure.
Because the metallic conduit must be safely grounded, it is also
necessary that the electrical enclosure afford a low ohmic
resistance to the passage of fault current to the conduit, or to
maintain bonding continuity. This presents no problem where the
explosion-proof electrical enclosure is constructed of metal, but
requires that special provision be made to maintain this continuity
if the enclosure is constructed of plastic materials which are
relatively non-conducting.
There is a further requirement in hazardous location areas that the
chance of sparks initiating due to the discharge of static
electricity be kept to a minimum. For an enclosure constructed of
plastic, great care must be taken to assure that all exposed
metallic parts be effectively grounded to prevent the build-up of
static electric charges on these exposed metallic parts which could
lead to a static spark under certain conditions that would initiate
an explosion outside of the box. While it would be possible to
control the resistivity of the plastic material in such a fashion
that dangerous charges would be bled from exposed metal parts, the
requirements in formulating a plastic material with these
characteristics might conflict with optimum corrosion resistance
for the material or with the advantage that a relatively
non-conducting plastic enclosure has in comparison with metal for
enclosing live electrical parts.
For instance, great care must be taken in metallic enclosures to
isolate live electrical parts from the walls of the metallic
enclosure to assure that electrical faults will not occur and
destroy the effectiveness of the equipment installed within the
enclosure. Relative non-conducting plastic is much safer in this
respect, as is obvious to those skilled in the art.
An object of this invention is to provide an explosion-proof
enclosure substantially constructed of plastic of a flat joint
construction which will not deteriorate due to repeated explosions
within the enclosure.
Another object is to provide an electrical enclosure which will
maintain continuity of the conduit ground from each conduit entry
way to each other conduit entry way.
A further object is to provide an explosion-proof enclosure with
all exposed metal parts conductively bonded to each other to
minimize the danger of a spark due to static electricity.
Another object is to provide an explosion-proof enclosure of
materials specifically engineered for maximum resistance to the
effects of corrosion for the most severe applications.
A further object is to provide an explosion-proof enclosure that
may be manufactured at a minimum cost.
Further objects and advantages will appear from the following
description.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a section through an embodiment of my invention
comprising a box and cover;
FIG. 2 is a plan view of the box as seen at line 2--2 of FIG.
1;
FIG. 3 is a partial section as viewed at line 3--3 of FIG. 2;
and
FIG. 4 is an elevational view of the stud which is incorporated in
each corner of the box.
DESCRIPTION OF SPECIFIC EMBODIMENT
The following disclosure is offered for public dissemination in
return for the grant of a patent. Although it is detailed to ensure
adequacy and aid understanding, this is not intended to prejudice
that purpose of a patent which is to cover each new inventive
concept therein no matter how others may later disguise it by
variations in form or additions or further improvements.
In the disclosed embodiment there is an explosion-proof electrical
enclosure consisting of a box, generally 10, and a cover, generally
11. The box and cover are primarily plastic. The plastic can be a
polyester resin such as that sold by Rostone Corporation under the
trademark Rosite or it could be a polymerized acetaldehyde resin
such as that sold under the trademark Delrin. Others of the various
available plastic materials also could be employed.
The box has two annular metal inserts 12. These are internally
threaded at 13 to receive and engage the threads on the ends of
electrical conduit 14. At each of the four corners of the box is a
metallic insert 15 having internal threads 16 at each end thereof.
The face of the box which mates with a corresponding face of the
cover is defined by a metallic flange 18. This flange completely
encircles the internal opening 19 defined by the box. This flange
has integral portions 20 which extend down in juxtaposition to the
internal plastic walls 21. At the ends of the opening 19 those
portions extend farther down and abut against the top of the
inserts 12 as seen at 22. Metallic screws 23 extend through these
portions 22 and are threaded into inserts 12.
The cover likewise has a metallic flange 26 which encircles the
internal opening 27 defined by the cover. This flange 26 is a
peripheral part of a dished member having a flat base 28 which
extends across the top interior of the cover 11. The cover has
threaded openings 29 and 30 for holding electrical components such
as a push-button switch 32, a signal light, etc. Switch 32 has a
threaded mounting member 33 which is screwed into opening 30. A
locking ring or an escutcheon plate 34 is threaded onto member 33
on the outside of the cover. Projecting from mounting member 33 is
a push-button 35 for actuating the switch (or it could be the lens
of a signal light). At least part of the case of switch 32 where it
abuts base 28 is of metal to thereby ground the switch. Screw
terminals 36 are provided for making electrical connections to the
wiring within the box, as for example to wires 37 which are brought
in through conduit 14.
The cover is held to the box by screws 39 which project through
openings in the corners of the cover, with their heads on the
exterior of the cover, and are screwed into the adjacent ends of
inserts 15. Various means can be employed for mounting the box on
walls, etc. One such means would be to use mounting brackets 40
which are attached to the inserts 15 by means of screws 41.
Brackets 40 have openings 42 through which mounting screws, bolts,
etc., may project for affixation to a wall, etc.
The box and cover can be formed by injection molding. The inserts
12 and 15 would be placed in the mold before the plastic was
injected therein. Thus, they would be imbedded in the plastic when
it was injected into the mold. To make this imbedment secure, the
inserts 12 and 15 have grooves 45 and 46, respectively, which
define ridges therebetween. If desired, each of them may be
provided with longitudinal grooves or ridges to also obtain a
rotational interlock between the plastic and the metal.
Alternatively, the inserts 12 and 15 could be glued in place after
molding of the box by means of properly selected resins. The flange
18 could also be affixed to the inserts 12 by screws 23 and
positioned in the mold at the same time as the inserts.
Alternatively, this assembly of the flange could be completed after
the box was otherwise molded. Similarly, the deep dish member which
defines flange 26 and base 28 could be positioned in the cover mold
before the cover was molded therein. Alternatively, these could be
formed as separate parts and subsequently assembled.
The flanges 18, 26 do not extend completely to the exterior of the
enclosure. Thus there are plastic portions, e.g. portion 48 on box
10, which surround the metal flanges 18, 26. For all practical
purposes, these exterior plastic faces (e.g. 48) are coplanar with
the flanges 18, 26. The two define a plane which forms the
demarcation between the box and the cover and which is at
substantially right angles to the internal walls which define
openings 19, 27. While the exterior plastic faces (e.g. 48) are, in
the described embodiment, coplanar with the metal flanges (e.g. 18)
this need not be exact.
While, from the drawings, it might appear that the box and cover at
this plane are in complete face to face contact, this is not
actually the case. Due to surface irregularities there are minute
fluid passageways which exist between the box and the cover at this
plane. These passageways provide vents for the escape of the gases
from the interior of the enclosure, which gases occur by reason of
an explosion within the enclosure. To retard erosion of these
passageways, the flanges 18, 26 are formed of an erosion resistant
metal, as for example, stainless steel, hard bronze, etc. The
components that form these flanges can be manufactured quite
inexpensively. They can be produced of relatively thin metal,
particularly as compared to the thickness of metal that would be
required for dimensional stability were the complete box wall to be
formed of such a metal. They can be manufactured by an inexpensive
metal stamping process. The inserts 12 and 15 need not be of any
special metal, but can be soft iron. Quantities thereof can be
rapidly produced on a screw machine. Of course, the plastic molding
of the box and cover is a comparatively inexpensive manufacturing
process on a mass production basis.
Assuming that conduit 14 is grounded in a conventional manner, the
electrical components (e.g. switch 32) mounted within the box will
automatically be grounded. Thus the screws 23 and the depending
portions of flanges 18 (e.g. portion 22) form an electrical ground
connection from the inserts 12 to the flange 18. There will be
sufficient metallic contact between flanges 18 and 26 so as to
provide an electrical connection therebetween. And as previously
described, metallic parts of the case of switch 32 will bear
against base 28 so as to form an electrical ground connection
therebetween. Electrical ground connections also can be made within
the box by placing a wire under one of screws 23 and tightening the
screw down against the wire.
The erosion that occurs will primarily be at the inner edge of the
passageways, for example, along the line where the face of flange
18 meets the depending portion 22. Since the flange 18 extends a
substantial distance outwardly from this line of juncture, and
because the flange is formed of an erosion resistant material,
embodiments of the invention will have a long service life without
incurring the danger of the propagation of flame to the exterior of
the box.
In completing an installation using the illustrated embodiment, any
of the openings that are not employed for electrical components or
conduits would, of course, be closed with a threaded plug, Thus, if
a conduit were not inserted into the right hand insert 12 a plug
having pipe threads would be screwed into this opening in
engagement with threads 13. Similarly, if an electrical component
was not mounted in threaded opening 29 in the cover it likewise
would be closed with a plug.
The described embodiment is readily adaptable to receive various
sizes of conduits 14. Inserts 12 are comparatively larger in
diameter for the larger sizes of conduit 14 and comparatively
smaller in diameter for the smaller sizes of conduit. Portion 22 of
the box flange extends comparably lower in the box for the smaller
diameter inserts and descends less far in the box for the
comparatively larger diameter inserts. With the comparatively
smaller diameter inserts only one of mounting screws 23 per insert
will be employed as compared to the two illustrated. No other
changes are required in the manufacturing of boxes to receive
different sizes of conduit.
* * * * *